The present invention relates, in general, to semiconductor packages, and more particularly, to semiconductor packages for integrated circuits having an exposed die attach flag.
Extensive effort is ongoing in the electronics industry to reduce overall thickness of integrated circuit packages. At the same time, integrated circuit device density is increasing, creating a need for packages which dissipate heat readily. Commodity semiconductor devices have an integrated circuit or "chip" which is mounted on a die attach flag. Wires are attached from the chip to leads. The chip, die attach flag, wires, and portions of the leads are placed in a cavity defined by a cavity plate of a mold press, and are encapsulated in plastic. For integrated circuits, the plastic encapsulation is usually formed on both the top and bottom of the die attach flag.
The presence of plastic underneath the die attach flag significantly inhibits the flow of heat from the die attach flag to an external heatsink or circuit board on which the encapsulated device is mounted. Some semiconductor packages are manufactured with the die attach flag exposed to eliminate the thermal resistance caused by the plastic under the die attach flag. These packages use thick die attach flags, greater than one millimeter thick, so that during the encapsulation process the die flag can be mechanically clamped against the cavity of the mold press. Because the plastic is injected at high pressure during the encapsulation process, prior semiconductor packages required the die flag to be clamped to the cavity to prevent plastic from encroaching under the die flag during encapsulation.
One problem with die attach flags which are mechanically clamped to the cavity plate is that they must be quite thick to effectively prevent plastic from reaching the back surface of the die flag. Alternatively, thinner die flags can be used if they are clamped directly above by pins which firmly clamp the die flag while encapsulant fills the cavity, and retract as the encapsulant hardens. Either of these solutions is unworkable for thin, small semiconductor packages which require leadframes which are quite thin. Also, retracting pins are very complex, expensive, and troublesome in production volume molding applications.
What is needed is a semiconductor package that allows for minimal displacement of the die attach flag during encapsulation without using external clamping, so that a thin semiconductor package can be provided with low thermal resistance.